The present invention relates to an optical recording medium, and more particularly to a writable optical recording medium having a light transmissive substrate on which a light absorption layer for absorbing a laser beam and a light reflecting layer for reflecting the laser beam are formed.
In the prior art, an organic coloring matter such as cyanine type dye or phthalocyanine type dye is generally well known as a recording film of a writable optical recording medium.
A writing method for such an optical recording medium is generally carried out by converging laser beams to a micro-area of the recording film and then converting the light energy to the heat energy, thereby to denature, i.e. form a pit, the recording film. In order to smoothly perform the denaturing operation of the recording film, two sheets, each composed of a substrate and a recording film which is disposed on the substrate, are generally prepared so that the respective recording films are opposed to each other as structure of the recording medium, a so-called air-sandwich structure.
The laser beam utilized in a writing method for the optical recording mediums of the structure described above is irradiated through the transmissive substrate, thereby forming a light readably pit in the recording film. The laser beam for the reading for reproducing data stored in the recording film has an output power less than that of the laser beam for the writing. By using the laser beam for the reading, the contrast between a portion in which the pit is formed and a portion in which the pit is not formed is read out as an electric signal.
In the prior art, there is also known a recoding medium of a so-called ROM (Read Only Memory)-type in which data is preliminarily recorded, being different from the above described type recording medium. Such ROM-type recording medium has been widely utilized in practical use in an audio record and information processing technology and is not provided with a writable recording film. Namely, prepits and pregrooves corresponding to the data to be reproduced are preliminarily formed on a plastic substrate by a press forming method and a reflecting layer made of metal such as gold (Au), silver (Ag), copper (Cu), aluminium (Al) and the like is formed on the plastic substrate. A protective layer is further formed on the reflecting layer. A typical example of such ROM-type recording medium is a compact disk so-called CD. The specification of recording and reading signals of the CD is standardized, and a reproducing device of the CD has been widely utilized in accordance with this standard as a compact disk player, i.e. a CD player.
The writable optical recording medium is similar to the CD in their shapes and in the utilization of the laser beam. Accordingly, it is a strong desire to develop the writable recording medium which is directly utilized for the CD player without making any change or modification.
However, the conventional writable recording medium having no air-sandwich structure, which is merely provided with a light reflection layer on the conventional recording film, has a reflection coefficient of the laser beam and a modulation rate insufficient for the CD standards. In addition, the durability against the optical degradation is also required.